Effects of underfeeding and of fish meal supplementation on forage digestion in sheep

Abstract

SUMMARYThe effects of severe food restriction and of protein supplementation on the apparent digestibility of a forage were studied on 12 wethers in 1992 at INRA, Theix. Ruminal digestion of the forage cell walls was studied in a second experiment carried out on four ewes fitted with rumen and duodenal cannulas, receiving the same diets as the wethers. The animals were fed on two different hay-based diets: hay only (58 or 25 g/kg BW0·75 per day) or hay supplemented with fish meal (48 or 20 g hay with 6·5 and 3·0 g fish meal respectively per kg BW0·75). The effects of fish meal supplementation on apparent digestibility of the hay cell walls were small, + 1·8 and + 2·3 units for neutral detergent fibre (NDF) and acid detergent fibre (ADF) respectively. In contrast, NDF and ADF digestibility was respectively 4·0 and 4·7 units higher at low than at high intake. Rumen fluid volume decreased by 2·4 1 with decreasing intake, while water concentration in rumen content increased slightly from 89·8 to 91·8%. The mean retention time of the hay particles in the foreguts increased concomitantly from 39·5 to 55·1 h. This was achieved by a higher retention time to a similar extent in both slow and fast compartments. The calculated in situ degradability of NDF and ADF increased respectively by 12·1 and 13·1 units with decreasing intake. This reflected the lower fractional outflow rate recorded at low intake. If a fractional outflow rate value of 4·1%/h (as measured in high intake diets) was applied to all dietary treatments, NDF and ADF in situ degradability would be greater at low than at high intake (31·7 v. 26·6% for NDF; 27·9 v. 22·1 % for ADF), indicating a higher microbial activity in restricted diets. Interactions between intake and fish meal supply were detected for in situ degradation parameters, indicating principally a reduction of the cell wall undegraded fraction in supplemented diets when intake declined. Ruminal fermentation was modified by the dietary treatments, the acetate: propionate ratio being higher at low intake. Fish meal increased concentrations of ammonia and of isoacids in rumen liquid. The relationship between apparent digestibility and in situ degradability of plant particles is discussed.